Ask GeoMan...

Can you please tell me the difference between chalk and limestone?
If they have the same chemical formula why are they called two
different things?

Limestone is a general name for a vide variety of sedimentary
rocks which are composed primarily of calcium carbonate.
Calcium carbonate comes from many sources, most of which have
a biological origin. Things like bones and teeth and shells are
generally made of this amazing stuff, and when the critter dies,
the calcium carbonate is returned to the earth for recycling
into new and wondrous forms.

There are many types of creatures on earth, and quite a few
of them (at least by the sheer number of individuals) are composed
of a single living cell. There are quite a few different types
of these primitive life forms. Many live in the ocean, and some
extract calcium carbonate from the seawater to make a protective
shell. One cell may not seems like much, but hey, it's all they've
got and worth taking care of. But sooner or later the creature
does one of two things: reproduce or die. Either way, the discarded
shell settles to the seafloor, where untold bazillions of them
pile up. Over the course of geologic
time, the layers of calcium carbonate are lithified and converted
into rock.

"Chalk" is a variety of "limestone" which
is composed primarily of the shells of single-celled, calcium
carbonate secreting creatures. Think about this next time you
use chalk - that white stuff all over your hands is actually
countless individual shells, each microscopic in size and the
former home of a living being.

Click here for more information
on primary sources of calcium carbonate.